This invention relates to image sensors for use in such devices as facsimile machines and scanners, and more particularly to image sensors that are small in size and capable of producing accurately read signals.
Among conventional image sensors, particularly, those of contact type, there is a TFT-driven image sensor, which projects image data of an original or the like on a one-to-one basis and converts it into an electric signal. In this case, the projected image is divided into a number of picture elements (light-receiving elements) and the electric charges generated at the respective light-receiving elements are temporarily stored in load capacitors by a block consisting of a predetermined number of light-receiving elements using thin film transistor switching elements (TFT) and read as electric signals sequentially and chronologically at speeds of from several hundreds of KHz to several MHz. In the TFT-driven image sensor, the operation of the TFTs allows only a single drive IC to read the image data, thus contributing to reducing the number of drive ICs.
The TFT-driven image sensor, whose equivalent circuit diagram is shown, e.g., in FIG. 6, comprises a line-like light-receiving element array 11 whose length is substantially the same as the width of an original, an electric charge transfer unit 12 consisting of a plurality of thin film transistors T.sub.N,n, whose number corresponds to that of light-receiving elements 11' on a one-to-one basis, and multilayer interconnection 13.
The light-receiving element array 11 is divided into a plurality (N) of blocks, each block consisting of a plurality (n) of light-receiving elements 11'. The plurality (n) of light-receiving elements 11' can be expressed equivalently as a photodiode PD.sub.N,n. Each light-receiving element 11' is connected to the drain electrode of each thin film transistor T.sub.N,n. The source electrode of each thin film transistor T.sub.N,n is connected to each common signal line 14 (n lines) and each load capacitor C.sub.n every light-receiving element block through the matrix-like connected multilayer interconnection 13. The gate electrode of each thin film transistor T.sub.N,n is connected to a gate pulse generating circuit (not shown) so that the thin film transistor can be energized every block. The photoelectric charges generated at each light-receiving element 11' are stored by the parasitic capacitor of each light receiving element and by the overlap capacitor between the drain and gate of the thin film transistor for a predetermined time period, and then are sequentially transferred to and stored by the load capacitor C.sub.n every block using the thin film transistor T.sub.N,n as a transfer switch. That is, when a gate pulse .phi.Gl from the gate pulse generating circuit (not shown) is applied to turn on thin film transistors T.sub.1,l to T.sub.1,n of a first block, electric charges generated and stored by the respective light-receiving elements 11' of the first block are transferred to and stored by the respective load capacitors C.sub.l to C.sub.n. The potentials of the respective common signal lines 14 are changed by the amount of electric charges stored by the load capacitors, and these changed voltage values are used to sequentially turn on analog switches SW.sub.l to SW.sub.n within the drive IC 15 thereby to draw out them to an output line 16 chronologically. By turning on thin film transistors T.sub.2,l to T.sub.2,n, through T.sub.N,l to T.sub.N,n of the second to Nth blocks by gate pulses .phi.G2 to .phi.Gn, the electric charges stored by the light-receiving elements are transferred every block and sequentially read. This sequential operation allows image signals to be obtained every line of the original in a main scanning direction, and the image signals covering the entire original can be obtained by repeating this operation, while feeding the original by an original feeding means (not shown) such as a roller (See: Japanese. Patent Unexamined Publication No. 9358/1988).
Further, as shown in FIGS. 6 and 7, the conventional load capacitors C.sub.n comprise an individual lower electrode portions 41 made of such a material as chromium (Cr) on a substrate below the multilayer interconnection 13 on the extension of the lines drawn from the source electrodes of the thin film transistors T.sub.1,l to T.sub.1,n in unitization with these lines; an insulating layer 42 arranged above these lower electrode portions 41 and made of such a material as polyimide, SiNx, and SiO.sub.2, which insulating layer being used as the insulating layer interposed between the upper and lower lines of the multilayer interconnection 13; and a belt-like common electrode portion deposited on the insulating layer 42 and made of such a material as aluminum (Al) thereby to form an upper electrode 43 portion which is, e.g., grounded to maintain a predetermined potential.
However, the image sensor thus constructed requires each load capacitor C.sub.n whose capacitance is so many as several hundreds of pF that in the case where such a general insulating material as polyimide is used as a dielectric of the load capacitor C.sub.n, the area of the load capacitor must be large, thereby presenting the problem of having to increase the size of the image sensor.
Further, in such an image sensor as described above, the load capacitor C.sub.n is so constructed that the lower electrode 41 is used as the individual electrode and connected to the thin film transistor T.sub.N,n via the matrix-like multilayer interconnection 13 and the upper electrode 43 is connected to the earth as the belt-like common electrode. In order to increase particularly the capacitance of the load capacitor, the film thickness of the upper and lower electrodes must be decreased. However, as shown in FIG. 7, with the load capacitor constructed as described above, the irregularities formed by the lower electrodes 41 cause the surface of the upper common electrode 43 to be likewise irregular and thus make it difficult to flatten the surface. As a result, not only each interface of the irregularities becomes subject to shorts or breakage but also the non-flat surface of the upper common electrode 43 makes the designed allowable capacitance no longer valid, thus presenting the problem that reading accuracy is not satisfactory.